Boundary conditions (Acoustics BEM)

• Acoustic pressure boundary conditionsYou can use pressure boundary conditions to model free surface conditions, an opening in a cavity, and so on. You can define pressure boundary conditions in the frequency domain.You can apply acoustic pressure to the acoustic fluid elements with the Enforced Acoustic Pressure boundary condition.For more information, see Defining acoustic sources and loads (Acoustics BEM).

• Acoustic velocity boundary conditionsYou can use velocity boundary conditions to represent rigid planes, perfectly reflective surfaces, hard surfaces, or ground. You can apply acoustic velocity to the acoustic fluid elements with the Normal Acoustic Velocity boundary condition.For more information, see Defining acoustic sources and loads (Acoustics BEM).

• Acoustic absorbers - modeling admittance/impedance conditionsAcoustic absorbers are simulation objects that you use to apply a specific admittance and impedance absorption property in specific locations. You define acoustic absorbers on the 2D elements of the acoustic model. You can specify the absorption properties of a surface in terms of either the normal acoustic impedance or normal acoustic admittance.Acoustic impedanceAcoustic impedance (Z) is given byZ = p/Vwhere Z is the ratio of acoustic pressure (p) and acoustic particle velocity (V).The normal impedance (Zn) is given byZn = p/Vnwhere Vn represents the normal particle velocity with respect to the surface. This normal velocity is a relative velocity, which is the difference between the acoustic particle normal velocity and the structural normal velocity of the panel.Acoustic admittanceAcoustic admittance (Y) is given byY = V/pwhere Y is the ratio of acoustic particle velocity (V) and acoustic pressure (p).The normal admittance (Yn) is given byYn = Vn/pwhere Vn represents the relative normal particle velocity with respect to the panel.In physical terms, an acoustic impedance or admittance is closely related to absorption properties. The relation between the reflection coefficient (Rn) for a normally incident sound field and the acoustic impedance (Zn) of a surface is:In general, an acoustic impedance or admittance is complex and frequency dependent. The product ρ*c, which is Zn, is the characteristic impedance. A surface with this impedance value has a reflection coefficient equal to zero (reflection free or anechoic termination).You can define multiple acoustic absorber objects.

• Combining boundary conditionsYou can combine the boundary conditions as described in the following table:Use case****Combinations of boundary conditionsVibrating and absorbing panels for all BEM modelsVelocity + admittance/impedance.Note: Imposed velocity is a structural normal velocity.Computed velocity is a fluid normal velocity.Combination for indirect BEMDifferent velocities or admittance/ impedance on both sides.Pressure on one side and velocity on other side.Other combinationsVelocity and pressure on the same side.Pressure and admittance/impedance on the same side.Different pressures on both sides.For more information, see Defining acoustic sources and loads (Acoustics BEM).

• Solution subcasesYou create subcases to define loads in the solution.The following subcases types are available:Subcase – Acoustic ResponseSubcase – Vibro-Acoustic ResponseYou can create multiple subcases in the solution.

• Automatic generation of velocity boundary conditionsIn an indirect BEM vibro-acoustic solution, you can use load recipes to import structural vibrations as loading conditions to create an acoustic response solution.For more information on structural load boundary conditions, see Separated and aggregated boundary conditions.When you create a solution from load recipe, the software creates velocity boundary conditions automatically.For more information, see Create a solution from a load recipe.

• Acoustic sourcesYou can define an acoustic monopole source in the Acoustics BEM solver environment. An acoustic monopole is a pulsating sound source that radiates equally in all directions.For more information, see Defining acoustic sources and loads (Acoustics BEM).

• Symmetry conditions using infinite planesYou can reduce the model size by defining symmetry conditions using an infinite plane.Infinite planes are simulation objects that can be defined as one of the following.Rigid Plane (Symmetry, Zero Velocity) — Represents a zero normal velocity (Vn=0).An infinite plane that is used to represent a rigid and perfectly reflecting boundary condition defines a symmetric boundary condition. The infinite plane itself is not symmetric.Use a rigid plane to represent the acoustically reflective boundary between a fluid with lower characteristic acoustic impedance and a fluid or medium with much higher impedance, with the reference domain of interest being the one with lower impedance. A rigid plane defines identical acoustic conditions with respect to the plane. Defining a rigid plane is the easiest and computationally most efficient way to model a rigid surface.An example of a rigid plane is the concrete surface (hard surface) of a semi-anechoic chamber, where the sound radiates onto a nearly rigid surface.Pressure Release Plane (Anti-Symmetry, Zero Pressure) — Represents a zero acoustic pressure (p=0) or a free surface.A plane that is used to define a pressure-release surface defines an anti-symmetric boundary condition. The plane itself is not anti-symmetric.Use a pressure-release plane to represent the acoustically reflective boundary between a fluid with higher characteristic acoustic impedance and a fluid with much lower impedance, with the reference domain of interest being the one with higher impedance.An example of a pressure-release plane is the free surface of water to air above a submarine that radiates acoustic energy.Note: Acoustic boundary conditions are determined by how the acoustic fluid is supported at its boundaries.If a structure supports the acoustic fluid, such as the rigid walls of a tank, the acoustic boundary is a rigid plane or zero velocity condition.If no structure supports the acoustic fluid, such as the free surface of a fluid, the acoustic boundary is a pressure release or zero pressure condition. Examples include an air-to-water interface, or a large impedance mismatch between two fluids in the acoustic region.You can define up to three symmetric or anti-symmetric infinite planes in one solution. The infinite planes must be perpendicular to one of the three absolute coordinate system directions (X, Y, and Z), and therefore must be perpendicular to each other.Infinite plane cuts the beam sections at the base of a generator model

Where do I find it?

Creating an acoustic absorber

Application	Pre/Post
Prerequisite	A Simulation file as the work part and displayed partSimcenter 3D Acoustics BEM as the specified solverAcoustic or Vibro-Acoustic as the specified analysis type
Command Finder	Acoustic Absorber
Simulation Navigator	Right-click the Simulation Objects node of the active solution → New Simulation Object→Acoustic AbsorberorRight-click the Simulation Object Container node→New Simulation Object→Acoustic Absorber

Creating an infinite plane

Application	Pre/Post
Prerequisite	A Simulation file as the work part and displayed partSimcenter 3D Acoustics BEM as the specified solverAcoustic or Vibro-Acoustic as the specified analysis type
Command Finder	Infinite Plane
Simulation Navigator	Right-click the Simulation Objects node of the active solution → New Simulation Object→Infinite PlaneorRight-click the Simulation Object Container node→New Simulation Object→Infinite Plane

How do I

Define forcing frequencies (Acoustics BEM)

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Defining acoustic sources and loads (Acoustics BEM)

Assigning frequencies from all loads to subcases

Distributed Acoustic Plane Waves loading for indirect acoustic and vibro-acoustic analyses (Acoustics BEM)

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Boundary conditions (Acoustics BEM), Simcenter 3D 2021.1 Series

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